This invention relates to retro-reflective sheet material (also known as reflex reflectors) and to a method of preparing the same.
Retro-reflective material is well known and widely used in the prior art, mainly in connection with road signs and automobile licence plates and the like. The material is capable of reflecting an incident beam of light back in the general direction of the light source. When, for example, a car headlight illuminates a road sign bearing such material in otherwise dark surroundings, the sign reflects a greater amount of light towards the driver than surrounding objects and is therefore clearly visible.
Known retro-reflective materials fall into two general types. The first type consists of spherical glass beads adhered to the surface of a transparent organic coating applied over a reflective metal (or other) substrate but only partially imbedded in it so that a glass-air interface is presented to incoming light. This type of material does not function as a retro-reflector when the surface is wet with water.
The second type of material consists of high refractive index (about 1.9 and greater) glass beads suspended within a relatively thick film of a transparent organic coating (plastic film) applied over a reflective metal substrate. Such material retains its retro-reflective properties when wet.
Theoretical considerations dictate that the most effective retro-reflectors of the second type can be formed by positioning the glass beads at just the right distance from conforming segments of concave spherical mirrors of corresponding size located behind the individual beads. This optimum distance will vary depending on the diameter of the beads, their refractive index, and the refractive index of the medium in which they are suspended.
Various known retro-reflectors embody such a structure but have generally been produced in the form of flexible tapes or sheets which are adhered to the desired object, such as a road sign. The step of adhering the tape to the object can be time consuming and uneconomical, and the tape may peel from the object after a period of exposure to the elements.
One example of known retro-reflectors is disclosed in U.S. Pat. No. 2,407,680 issued on Sept. 17, 1946 and assigned to Minnesota Mining and Manufacturing Company. This patent was one of the first to disclose the second type of structure referred to above employing high refractive index spheres, and it is to be noted that it suggests the use of a polished metal surface as the back reflector with the beads spaced an optimum distance therefrom. The patent also suggests the formation of concave mirrors in a reflective surface formed by a reflective binder layer for the beads.
U.S. Pat. No. 2,543,800 issued on Mar. 6, 1951 and assigned to Minnesota Mining and Manufacturing Company discloses a retro-reflector in which the beads are spaced a small distance from corresponding reflector surfaces formed by pressing the beads partially into a moldable cushion layer having a reflective surface coating containing metallic flake pigment particles, the beads being spaced from the cushion layer by a thin film which may contain a transparent pigment. After the pressing operation the plastic layers are cured. One disadvantage of such retro-reflectors is that the reflector surfaces formed by the cushion layer are not as reflective as a polished metal surface and therefore light is lost at these surfaces.
U.S. Pat. No. 3,922,433 issued on Nov. 25, 1975 and assigned to Aluminum Company of America relates to partially embedding the spherical glass beads into a metallic coating while it is in the molten condition. This invention is an attempt to form a retro-reflective surface directly on a substrate made of a hard material, such as a road sign, without first forming a flexible tape to be adhered thereto. An iron-base substrate is dipped into a molten bath of aluminum, zinc, tin, lead or alloys thereof and is sprayed with the glass beads by an air gun as the substrate is withdrawn from the bath and the coating is still molten. This method has the disadvantages that it is expensive and the beads are not spaced from the reflective surface as is required for the optimum retro-reflection.
There is therefore a need for a method of producing a retro-reflective surface directly onto hard metal substrates, which method permits the beads to be spaced by the optimum distance from conforming concave reflective surfaces.
The formation of a plastic layer containing the glass beads and the pressing of the beads into the metal substrate has been contemplated but, because of the relative hardness of the substrate surface, it has been found that the beads tend to shatter and the layer of plastic between the beads and the substrate surface tends to become attenuated or damaged when the plastic layer is rolled with sufficient force to cause the beads to indent the substrate surface. Moreover, the beads tend to become mis-aligned with the indentations in the surface so that a useless product is produced.
It has now unexpectedly been found that a product of high quality can be produced by indenting the substrate surface by applying pressure to the glass beads when a platen is located between the surface of the layer containing the glass beads and a roller or the like used for applying said pressure.
Thus, according to one aspect of the invention, there is provided a method of forming a retro-reflective surface on an indentable metal substrate, comprising the steps of; (a) forming on said indentable metal substrate a layer of transparent organic polymeric material having a mono-layer coating of substantially spherical glass beads of high refractive index; (b) covering said beads with a platen having substantially no tendency to adhere to the glass beads under the pressures encountered; (c) applying sufficient pressure to said platen and indentable metal substrate to cause the glass beads to indent the surface of said indentable metal substrate; and (d) covering said beaded layer with a further layer of transparent organic polymeric material; said transparent organic polymeric material formed on the indentable metal substrate in step (a) being suitable to withstand the pressure of step (c) without substantial crazing, cracking or attenuation, and the thickness of said layer being sufficient to space said beads from said substrate by a predetermined distance suitable for retro-reflection after step (c).
According to another aspect of the invention there is provided a retro-reflector comprising an indentable metal substrate, a layer of transparent organic polymeric material overlying a surface of the substrate, a mono-layer of glass beads of high refractive index separated from said substrate by said layer of transparent organic polymeric material, and a further layer of transparent organic polymeric material overlying said mono-layer of glass beads, the surface of said substrate having indentations conforming to the adjacent glass beads, and the separation of the glass beads from the conforming indentations being sufficient for retro-reflection.
When a platen is used in the indentation step, it is found that, despite the large pressure or force required to force the glass beads to indent the substrate surface, the glass beads remain largely undamaged and the layer between the beads and the substrate is not significantly damaged or compressed.
The effectiveness of the platen is quite unexpected because the damage to the glass beads and misalignment of the beads with the conforming convex mirrors encountered in original attempts to form retro-reflective surfaces on hard substrates was believed to be due to the load required to force each bead to indent the substrate material via the intervening polymer layer. If a platen is used, the beads must be subject to the same loading in order to cause proper indentation, so it was unreasonable to expect a platen to prevent damage to the beads.
The platen may take the form of any resilient or deformable material having substantially no tendency to adhere to the glass beads during the indentation step. The material of the platen should be sufficiently soft to be indented either elastically or plastically by the glass beads during the indenting step, but of course should be capable of transmitting sufficient force to the beads to cause the necessary indentation of the substrate.
Thus the platen may be a plate or sheet of aluminum or other metal. It has also unexpectedly been found that the platen can be a thin foil or web of metal or paper or similar material. Such thin foils or webs are particularly advantageous because they can be withdrawn from a roll of the material and passed virtually continuously through a roll mill with the beaded substrate. This makes the manufacture of the retro-reflective substrate economical particularly as the foil or web-like platens are themselves inexpensive.
Although the use of a platen eliminates damage to the glass beads during the indentation step in the case of most metal substrates, some substrates (especially ferrous metals) are so hard that the loading required to produce indentation cannot be transmitted by the beads without considerable damage, even when a platen is used. These substrates can easily be identified from hardness tables and from simple trial and error. It is not possible to provide a maximum hardness limit beyond which the method of the invention cannot be effectively operated, because the limit varies somewhat with the particular type of pressing equipment, glass beads, platens, operating speeds, etc. However, as stated, the useful metal substrates will be readily identifiable by persons skilled in this technology. The term "indentable metal substrate" is used throughout this specification to refer to those substrates which can be used in the method of the invention, i.e., those having a hardness below the practical maximum hardness limit in the particular operating conditions.
Very hard metals can be provided with a retro-reflective surface if the hard metals are first coated or "clad" with a softer metal because only the hardness of the surface layer of the substrate is important in the indentation step. The term "indentable metal substrate" therefore includes such structures.
It is also to be noted that the term "transparent" is used in a wide sense throughout this specification and is intended to include materials that are sometimes referred to as semi-transparent. The important point is that the various layers overlying the substrate, and even the glass beads themselves, must be capable of transmitting sufficient light for the structure to function effectively as a retro-reflector. Any layer of material (e.g., pigment or dye) capable of transmitting sufficient light to achieve this function is considered to be "transparent" in the context of this invention.